Japanese Patent Application Publication No. Heisei 3-216966,published on Sep. 24, 1991 (which corresponds to U.S. Pat. No. 4,983,471, issued on Jan. 8, 1991) exemplifies a previously proposed fuel cell system. In that fuel system, a catalytic reforming material is disposed in a channel extended from a gas mixer to a solid oxide fuel cell (SOFC) and part of a spent fuel containing steam is supplied to the gas mixer to be mixed with a reformable fuel (a fresh feed fuel). Thereafter, the mixed gas is supplied to a reformer and the SOFC.